Thassarian at the Wailing Ziggurat wants you to go to the Temple City of En'kilah and retrieve High Priest Talet-Kha's Scroll from the Spire of Pain, High Priest Naferset's Scroll from the Spire of Decay and High Priest Andorath's Scroll at the Spire of Blood.
The lich has spoken, [player]. The high priests of En'kilah hold the secret to entering Naxxanar.
Go to the Spire of Blood, the Spire of Pain and the Spire of Decay and slay each of the priests.
Return once you've obtained the three scrolls.